Two examples are available in the examples directory:
hello-tcl, hello-tcl-tk.

Before marking strings as internationalizable, substitutions of variables
into the string need to be converted to format applications. For
example, "file $filename not found" becomes
[format "file %s not found" $filename].
Only after this is done, can the strings be marked and extracted.
After marking, this example becomes
[format [_ "file %s not found"] $filename] or
[msgcat::mc "file %s not found" $filename]. Note that the
msgcat::mc function implicitly calls format when more than one
argument is given.